Naphthenic acids (NAs) are mixtures of several cyclopentyl and cyclohexyl carboxylic acids with molecular weights of 120 to well over 700 atomic mass units. The main fractions are carboxylic acids with a carbon backbone of 9 to 20 carbons. McKee et al. claim that "naphthenic acids (NAs) are primarily cycloaliphatic carboxylic acids with 10 to 16 carbons", although acids containing up to 50 carbons have been identified in heavy petroleum.
Nomenclature Naphthenic acid can refer to derivatives and isomers of naphthalene carboxylic acids. In the petrochemical industry, NA's refer to alkyl carboxylic acids found in petroleum. The term naphthenic acid has roots in the somewhat archaic term "naphthene" (cycloaliphatic but non-aromatic) used to classify hydrocarbons. It was originally used to describe the complex mixture of petroleum-based acids when the analytical methods available in the early 1900s could identify only a few naphthene-type components with accuracy. Today "naphthenic" acid is used in a more generic sense to refer to all of the carboxylic acids present in petroleum, whether cyclic, acyclic, or aromatic compounds, and carboxylic acids containing heteroatoms such as N and S. Although commercial naphthenic acids often contain a majority of cycloaliphatic acids, multiple studies have shown they also contain straight chain and branched aliphatic acids and aromatic acids; some naphthenic acids contain >50% combined aliphatic and aromatic acids. Salts of naphthenic acids, called naphthenates, are widely used as hydrophobic sources of metal ions in diverse applications.
Classification Naphthenic acids are represented by a general formula CnH2n-zO2, where n indicates the carbon number and z specifies a homologous series. The z is equal to 0 for saturated, acyclic acids and increases to 2 in monocyclic naphthenic acids, to 4 in bicyclic naphthenic acids, to 6 in tricyclic acids, and to 8 in tetracyclic acids. Crude oils with total acid number (TAN) as little as 0.5 mg KOH/g acid or petroleum fractions greater than about 1.0 mg KOH/g oil usually qualify as a high acid crude or oil. At the 1.0 mg/g TAN level, acidic crude oils begin to be heavily discounted in value and so are referred to as opportunity crudes. Commercial grades of naphthenic acid are most often recovered from kerosene/jet fuel and diesel fractions, where their corrosivity and negative impact on burning qualities require their removal. Naphthenic acids are also a major contaminant in water produced during the extraction of oil from Athabasca oil sands.
Sources and occurrence Naphthenic acids are extracted from petroleum distillates by extraction with aqueous base. Acidification of this extract acidic neutralization returns the acids free from hydrocarbons. Naphthenic acid is removed from petroleum fractions not only to minimize corrosion but also to recover commercially useful products. Some crude oils are high in acidic compounds (up to 4%).
Naphthenic acid corrosion The composition varies with the crude oil composition and the conditions during refining and oxidation. Fractions that are rich in naphthenic acids can cause corrosion damage to oil refinery equipment; the phenomenon of naphthenic acid corrosion (NAC). Crude oils with a high content of naphthenic acids are often referred to as high total acid number (TAN) crude oils or high acid crude oil (HAC).
Rare earth separation Naphthenic acid was first discovered as a method for yttrium extraction from lanthanide elements by the Bureau of Mines in the USA in 1964. The application of naphthenic acid in rare earth separation was identified by the Changchun Institute of Applied Chemistry in 1974. Between 1974 and 1975, the Nanchang 603 Factory collaborated with the Changchun Institute and other units to successfully develop a third-generation extraction process for yttrium oxide. This process, which utilized naphthenic acid for one-step extraction of high-purity yttrium oxide, was put into operation in 1976. Naphthenic acid has been successfully used in industrial rare earth separation due to its advantages of low cost and abundant availability. In solvent extraction, the H+ released from naphthenic acid decrease the aqueous acidity, thereby limiting the positive extraction reaction and leading to an unsatisfied extraction efficient. To address this issue, alkaline substances (bases) are used to saponify naphthenic acid before extraction. This step helps avoid the generation of H+, effectively controlling the equilibrium acidity and improving extraction efficiency. However, this process produces a significant amount of ammonium nitrogen wastewater, which necessitates additional wastewater treatment after extraction. Currently, research is ongoing to minimize the use of bases and maximize separation efficiency, including adding additives and/or ionic liquids.
… excerpt ends here. Continue reading the full article.
